In coating the body of an automobile or any other substrate, a coating film structure is generally used which includes an undercoating film made of an anticorrosive electrodeposition paint, an intermediate coating film having undercoat concealability and formed on the undercoating film, and a top coating film (a base coating film and a clear coating film) superimposed on the intermediate coating film. In view of savings in resources and other factors, an attempt has also been made to omit such an intermediate coating film and superimpose a top coating film directly on an undercoating film. For example, an attempt has been made to form a base coating film having undercoat concealability on a cationic electrodeposition coating film and form a clear coating film on the base coating film.
An attempt to provide new design properties to an automotive coating film by adjusting the refractive indices of base and clear coating films has been conventionally known. For example, Patent Document 1 discloses that the refractive index of a clear coating film is set to be 1.522, and the difference in refractive index between a base coating film and the clear coating film is set to be greater than or equal to 0.015, thereby allowing the brilliance of a coating to vary among the angles at which a viewer sees the coating. Patent Document 1 further discloses that an acrylic resin having a t-butyl methacrylate content of greater than or equal to 50 mass % has a low refractive index, and an acrylic resin having a styrene content of greater than or equal to 40 mass % has a high refractive index.
Patent Document 2 discloses that ZnO2 nanoparticles increase the refractive index of a resin if they are dispersed into the resin, and that a resin thus obtained is used as a paint composition. Patent Document 3 discloses that a clear-coated body panel of an automobile is coated with a silane coating composition containing silica nanoparticles to provide antireflection surface properties.